The invention relates to a method of measuring properties of a moving web, the method comprising arranging a sensor on a first side of the moving web, the sensor comprising at least one coil, arranging a counterplate on a second side of the web, and feeding electric current to the coil.
The invention further relates to an apparatus for measuring properties of a moving web, the apparatus comprising a sensor, a counterplate and a control device, the sensor comprising at least one coil, the control device being adapted to feed electric current to the coil, the sensor being arranged on a first side of the moving web, and the counterplate being arranged on a second side of the moving web.
Measurements of properties of a moving paper web constitute the basis for controlling a paper machine. Continuous monitoring of properties, e.g. the caliper of paper, as the web moves, ensures that paper of the right quality is produced. The measurements serve to detect deviations in the web, and as a result, commands regarding necessary changes may be given to the actuators of the paper machine in order for the quality criteria to be fulfilled.
It is known to continuously measure the caliper of paper by apparatuses whose measuring heads are in contact with the paper web. In addition, U.S. Pat. No. 4,791,367 discloses a gauge for measuring the caliper of a web. The gauge comprises an upper head having a U-shaped magnetic pole piece whose legs are provided with windings. A lower head to be arranged on the opposite side of the paper web to be measured comprises a passive magnetic circuit which is a magnetic or ferrite plate. The magnetic flux established by the windings extends through the pole piece through the paper to the ferrite plate. The gap between the pole piece and the ferrite plate varies as the caliper of the paper web varies. The circuit inductance used to define the caliper of the paper web also varies simultaneously.
U.S. Pat. No. 4,528,507 discloses a solution for measuring properties of a moving web by sensors arranged on different sides of the web. One of the sensors is provided with a coil, the sensor on the opposite side of the web being provided with a counterplate. As the gap between the coil and the counterplate varies, the density of the magnetic flux established by the coil varies, and the caliper of the web is measured by detecting the variation in the density.
German Offenlegungsschrift 28 29 264 discloses a measuring apparatus, in which a sensor including a coil is arranged on both sides of the web. The caliper of the web is measured by defining the amplitude of the magnetic field established by the coils.
In all the above solutions, the sensor coils, typically made from copper wire, and the counterplates, e.g. of iron or ferrite, are heavy, making the sensors heavy and difficult to move flexibly along formations on the surface of paper. Furthermore, because the construction is heavy, the sensors are also very poorly adapted to float by air bearing, completely clear of the surface of paper.
It is an object of the present invention to provide a method and an apparatus for avoiding the above drawbacks.